An R—Fe—B based sintered magnet, as represented by an Nd—Fe—B based sintered magnet, has high magnetic characteristics, and thus is used today in various fields. However, because an R—Fe—B based sintered magnet contains a highly reactive rare earth element, R, it is easily oxidized and corroded in the atmosphere. When it is used without any surface treatment, the corrosion progresses from the surface by the existence of small amounts of an acid, an alkali, moisture, or the like, whereby rusting occurs causing deterioration or fluctuation in the magnetic characteristics. Further, there is a risk that rust is dispersed and contaminates peripheral parts when such a rusted magnet is incorporated into a device such as a magnetic circuit. Thus, with the purpose of giving the corrosion resistance to an R—Fe—B based sintered magnet, a method to form a plating film on the surface of a magnet is widely employed, as it is well known.
As the plating film formed on the surface of an R—Fe—B based sintered magnet, for example, a copper plating film, a nickel plating film, and the like are mentioned. These plating films can be formed by an electrolytic plating treatment or a non-electrolytic plating treatment, but, in the case when any of the plating films is formed, cleaning (acid cleaning) using an inorganic acid or an organic acid to remove processed deformed layer and sintered deformed layer existing on the surface of the magnet is conducted as a pretreatment before the plating treatment. After this, the removal of infusible residues, which are attached to the surface of the magnet by the acid cleaning and are called smuts, is conducted because a plating film excellent in adhesiveness cannot be formed when the plating treatment is conducted to the magnet, to which smuts remain attached.
As the method for removing smuts attached to the surface of the R—Fe—B based sintered magnet after the acid cleaning, and forming a plating film excellent in adhesiveness, for example, Patent Document 1 proposes a method: in which the magnet is placed in a barrel for plating; an electrolytic treatment is conducted with rotating the barrel in an alkaline electrolytic solution; and then a plating treatment is conducted without taking the magnet out of the barrel. In this method, smut removal is conducted using the force associated with the desorption of oxygen gas or hydrogen gas, which generates from the surface of the magnet by the electrolytic treatment from the surface of the magnet. The method can be appreciated in that the plating treatment can be conducted without the necessity of transferring the magnet after smuts are removed. However, by the analysis of the present inventors, it was found that, in this method, an oxide film or a hydroxide film is thought to be formed on the surface of the magnet with the generation of oxygen gas; thus, it is difficult to form a plating film excellent in adhesiveness on the surface of the magnet. This tendency is remarkable, especially when a plating treatment using a highly alkaline plating bath is conducted. Further, Patent Document 1 describes a method to remove smuts by conducting acid cleaning of the magnet placed in a mesh basket and then subsequently conducting ultrasonic cleaning, as prior art. This method however, does not have a sufficient effect of removing smuts and causes trouble because it is necessary to transfer the magnet from the mesh basket to a barrel for plating for the plating treatment, as described in Patent Document 1.